Long Term Evolution (LTE) is a radio access technology standardized by 3rd Generation Partnership Project (3GPP). LTE only supports a Packet Switched (PS) domain and is intended to eventually provide all services in the packet switched domain. LTE has no Circuit Switched (CS) domain in contrast to previous 3GPP systems, and circuit switched voice calls are therefore still supported by 2G and 3G networks on behalf of the LTE network. A user with an LTE-subscription that is making or receiving a voice call is hence redirected to the 2G or 3G network. Voice over LTE (VoLTE) is an Internet Protocol (IP) Multimedia Subsystem (IMS)-based specification intended for providing speech services over the packet switched domain.
To secure a successful roll out of speech services over LTE, superior performance, both with respect to integrity and retainability, is expected and regarded a corner stone. Naturally, operators as well as end users will have the same high quality expectations on VoLTE as they have on speech services in the 2G and 3G networks, e.g. Global System for Mobile Communications (GSM) and Wideband Code Division Multiple Access (WCDMA).
It has been observed that the handover procedure in LTE radio access networks is vulnerable to interference, i.e. when the interference in the radio access network increases the handover performance degrades. A handover failure resulting in a dropped call is a source of user dissatisfaction and it is therefore important to have a very high handover success rate. To meet the performance expectations of the LTE users, there is a need for solutions which increases handover robustness in LTE.